Golf ball

ABSTRACT

An object of the present invention is to provide a golf ball having excellent approach spin performance under a dry condition and excellent approach spin performance under a condition that there is grass between the golf ball and the club face. The present invention provides a golf ball comprising a golf ball body and a paint film formed on a surface of the golf ball body, wherein the paint film is composed of at least one layer, and an outermost paint film layer located at the outermost layer of the golf ball contains, as a base resin, a polyurethane obtained by a reaction between (A) a polyisocyanate composition and (B) a polyol composition, and the outermost paint film layer has a dynamic friction coefficient of from 0.75 to 1.5 measured with a dynamic friction tester under specific conditions.

FIELD OF THE INVENTION

The present invention relates to a golf ball comprising a paint film.

DESCRIPTION OF THE RELATED ART

A paint film is formed on a surface of a golf ball body. Conventionally, it has been proposed that the performance of the golf ball has been improved by improving the paint film and controlling the friction coefficient of the golf ball.

For example, JP 2006-75209 A discloses a golf ball comprising a golf ball body and a paint layer covering a surface of the golf ball body, wherein a resin component constituting the paint layer is a product cured by a polyamide curing agent, and the golf ball has a static friction coefficient of 0.22 or less.

JP 2013-176530 A discloses a golf ball comprising a golf ball body and a paint film formed on a surface of the golf ball body, wherein the golf ball has a friction coefficient of 0.35 or more and 0.60 or less calculated using a contact force tester.

In addition, it has been proposed to use a polyrotaxane as a component constituting the paint film of the golf bal. For example, JP 2017-209298 A discloses the use of the polyrotaxane. The polyrotaxane has a cyclodextrin, a linear molecule piercing through the cyclic structure of the cyclodextrin, and blocking groups located at both terminals of the linear molecule to prevent disassociation of the cyclodextrin wherein at least a part of hydroxyl groups of the cyclodextrin is modified with a caprolactone chain via a —O—C₃H₆—O— group.

SUMMARY OF THE INVENTION

However, the conventional golf balls do not necessarily have satisfactory spin performance on approach shots, and there is still room for improvement. In addition, in the conventional technologies, the spin performance on approach shots under a condition that there is grass between the golf ball and the club face was not investigated.

The present invention has been made in view of the abovementioned circumstances, and an object of the present invention is to provide a golf ball having excellent spin performance on approach shots under a dry condition and excellent spin performance on approach shots under a condition that there is grass between the golf ball and the club face.

The present invention that has solved the above problems provides a golf ball comprising a golf ball body and a paint film formed on a surface of the golf ball body, wherein the paint film is composed of at least one layer, and an outermost paint film layer located at the outermost layer of the golf ball contains, as a base resin, a polyurethane obtained by a reaction between (A) a polyisocyanate composition and (B) a polyol composition, and the outermost paint film layer has a dynamic friction coefficient of from 0.75 to 1.5 measured with a dynamic friction tester under following conditions:

<measurement conditions>

moving speed: 2 mm/s

load: 1.96 N

measuring item: dynamic friction average value in a moving distance of from 2 mm to 10 mm.

In the golf ball according to the present invention, the base resin of the outermost paint film layer contains a polyurethane, and the dynamic friction coefficient of the outermost paint film layer is controlled in a range of from 0.75 to 1.5. By such the configuration, the obtained golf ball is excellent not only in the approach spin performance under a dry condition but also in the approach spin performance under a condition that there is grass between the golf ball and the club face.

According to the present invention, a golf ball having excellent spin performance on approach shots under a dry condition and excellent spin performance on approach shots under the condition that there is grass between the golf ball and the club face is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a partially cutaway cross-sectional view showing a golf ball according to one embodiment of the present invention.

DESCRIPTION OF THE PREFERABLE EMBODIMENT

The present invention provides a golf ball comprising a golf ball body and a paint film formed on a surface of the golf ball body, wherein the paint film is composed of at least one layer, and an outermost paint film layer located at the outermost layer of the golf ball contains, as a base resin, a polyurethane obtained by a reaction between (A) a polyisocyanate composition and (B) a polyol composition, and the outermost paint film layer has a dynamic friction coefficient of from 0.75 to 1.5 measured with a dynamic friction tester under specific conditions.

(Paint Film)

The paint film may have either a single layered structure or a multiple layered structure. The outermost paint film layer located at the outermost layer of the golf ball contains a polyurethane as a base resin, wherein the polyurethane includes (A) a polyisocyanate composition and (B) a polyol composition as constituent components. When the paint film has a single layered structure, the single layered paint film contains the polyurethane as the base resin. When the paint film has a multiple layered structure, at least the outermost paint film layer contains the polyurethane as the base resin.

The amount of the polyurethane in the base resin of the outermost paint film layer is preferably 50 mass % or more, more preferably 70 mass % or more, and even more preferably 90 mass % or more. It is most preferable that the base resin of the outermost paint film layer consists of the polyurethane.

(Polyurethane)

The polyurethane contained in the outermost paint film layer will be explained. The polyurethane is obtained by a reaction between (A) the polyisocyanate composition and (B) the polyol composition. The properties of the paint film can be adjusted by changing the type or blended amount of the polyisocyanate component contained in (A) the polyisocyanate composition or the polyol component contained in (B) the polyol composition.

((A) Polyisocyanate Composition)

Examples of the polyisocyanate component of (A) the polyisocyanate composition include a compound having at least two isocyanate groups. Examples of the polyisocyanate include an aromatic polyisocyanate such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethylxylylenediisocyanate (TMXDI), and para-phenylene diisocyanate (PPDI); an alicyclic polyisocyanate or aliphatic polyisocyanate such as 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), hydrogenated xylylenediisocyanate (H₆XDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and norbornene diisocyanate (NBDI); and derivatives of these polyisocyanates. In the present invention, two or more polyisocyanates may be used as the polyisocyanate.

Examples of the derivatives of the polyisocyanate include an adduct-modified product obtained by a reaction between a diisocyanate and a polyhydric alcohol; an isocyanurate-modified product of a diisocyanate; a biuret-modified product of a diisocyanate; and an allophanate-modified product of a diisocyanate. The derivatives of the polyisocyanate from which free diisocyanate is removed are more preferable.

The adduct-modified product is a polyisocyanate obtained by a reaction between a diisocyanate and a polyhydric alcohol. Preferable examples of the polyhydric alcohol include a low molecular weight triol such as trimethylolpropane and glycerin. As the adduct-modified product, an adduct-modified product of hexamethylene diisocyanate is preferable, a triisocyanate (the following formula (1)) obtained by a reaction between hexamethylene diisocyanate and trimethylolpropane; and a triisocyanate (the following formula (2)) obtained by a reaction between hexamethylene diisocyanate and glycerin are more preferable.

Examples of the isocyanurate-modified product include an isocyanurate-modified product of hexamethylene diisocyanate, and a trimer of hexamethylene diisocyanate (the following formula (3)) is preferable.

Examples of the biuret-modified product include a trimerized product of a diisocyanate, and a trimer of hexamethylene diisocyanate (the following formula (4)) is preferable.

The allophanate-modified product is, for example, a triisocyanate obtained by further reacting a diisocyanate with a urethane bond formed by a reaction between a diisocyanate and a low molecular weight diol.

As the polyisocyanate component, the adduct-modified product is preferable, and the adduct-modified product of hexamethylene diisocyanate (preferably trimer) is more preferable. When the adduct-modified product of hexamethylene diisocyanate is used, the amount of the adduct-modified product of hexamethylene diisocyanate in the polyisocyanate component is preferably 30 mass % or more, more preferably 40 mass % or more, and even more preferably 50 mass % or more. It is also preferable that the polyisocyanate component consists of the adduct-modified product of hexamethylene diisocyanate.

In addition, as the polyisocyanate component, the adduct-modified product and the isocyanurate-modified product are preferably used in combination, and the adduct-modified product of hexamethylene diisocyanate (the trimer is preferable) and the isocyanurate-modified product of hexamethylene diisocyanate (the trimer is preferable) are more preferably used in combination. In this case, the mass ratio (adduct-modified product/isocyanurate-modified product) of the adduct-modified product to the isocyanurate-modified product is preferably 0.43 or more, more preferably 1.0 or more, and even more preferably 2.5 or more. If the mass ratio is 0.43 or more, the dynamic friction coefficient is greater and thus the spin rate on approach shots under a condition that there is grass between the golf ball and the club face increases.

In addition, when the adduct-modified product of hexamethylene diisocyanate and the isocyanurate-modified product of hexamethylene diisocyanate are used as the polyisocyanate component, the total amount of the adduct-modified product of hexamethylene diisocyanate and the isocyanurate-modified product of hexamethylene diisocyanate in the polyisocyanate component is preferably 70 mass % or more, more preferably 80 mass % or more, and even more preferably 90 mass % or more. It is also preferable that the polyisocyanate component consists of the adduct-modified product of hexamethylene diisocyanate and the isocyanurate-modified product of hexamethylene diisocyanate.

The amount (NCO %) of the isocyanate group of the polyisocyanate component is preferably 0.5 mass % or more, more preferably 1.0 mass % or more, and even more preferably 2.0 mass % or more, and is preferably 45 mass % or less, more preferably 40 mass % or less, and even more preferably 35 mass % or less. It is noted that the amount (NCO %) of the isocyanate group of the polyisocyanate component can be represented by 100×[mole number of isocyanate group in polyisocyanate×42 (molecular weight of NCO)]/[total mass (g) of polyisocyanate].

Specific examples of the polyisocyanate component include Burnock (Registered trademark) D-800, Burnock DN-950, and Burnock DN-955 available from DIC corporation; Desmodur (Registered trademark) N75MPA/X, Desmodur N3300, Desmodur N3390, Desmodur L75 (C), and Sumidur (Registered trademark) E21-1 available from Sumika Bayer Urethane Co., Ltd.; Coronate (Registered trademark) HX, Coronate HK, Coronate HL, and Coronate EH available from Tosoh Corporation; Duranate (Registered trademark) 24A-100, Duranate 21S-75E, Duranate TPA-100, Duranate TKA-100, and Duranate 24A-90CX available from Asahi Kasei Chemicals Corporation; and VESTANAT (Registered trademark) T1890 available from Degussa Co., Ltd.

((B) Polyol Composition)

Examples of the polyol component of (B) the polyol composition include a polyol compound having at least two hydroxyl groups in the molecule thereof. Examples of the polyol compound include a compound having a hydroxyl group at the terminal of the molecule, and a compound having a hydroxyl group at a part other than the terminal of the molecule. The polyol component may be used solely or as a mixture of at least two of them.

Examples of the compound having a hydroxyl group at the terminal of the molecule include a low molecular weight polyol having a molecular weight of less than 500 and a high molecular weight polyol having a number average molecular weight of 500 or more. Examples of the low molecular weight polyol include a diol such as ethylene glycol, diethylene glycol, triethylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, and 1,6-hexanediol; and a triol such as glycerin, trimethylolpropane, and hexanetriol. Examples of the high molecular weight polyol include a polyether polyol, a polyester polyol, a polycaprolactone polyol, a polycarbonate polyol, a urethane polyol, and an acrylic polyol. Examples of the polyether polyol include polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polyoxytetramethylene glycol (PTMG). Examples of the polyester polyol include polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA). Examples of the polycaprolactone polyol include poly-ε-caprolactone (PCL). Examples of the polycarbonate polyol include polyhexamethylene carbonate.

Examples of the compound having a hydroxyl group at a part other than the terminal of the molecule include a polyrotaxane having a hydroxyl group, and a hydroxyl group modified vinyl chloride-vinyl acetate copolymer.

The polyrotaxane having a hydroxyl group has a cyclodextrin, a linear molecule piercing through the cyclic structure of the cyclodextrin, and blocking groups located at both terminals of the linear molecule to prevent disassociation of the cyclic molecule. The polyrotaxane is viscoelastic, since the cyclodextrin molecule is movable along the linear molecule that penetrates the cyclodextrin in a skewering manner (pulley effect). Even if a tension is applied to the polyrotaxane, the tension is uniformly dispersed due to the pulley effect. It is noted that the polyol component in the present invention is not particularly limited, as long as it has a plurality of hydroxyl groups. Since a plurality of hydroxyl groups capable of reacting with an isocyanate group exist on the cyclic structure of the cyclodextrin, the polyrotaxane is the polyol component.

The cyclodextrin is a general term for an oligosaccharide having a cyclic structure. The cyclodextrin is, for example, a molecule having 6 to 8 D-glucopyranose residues being linked in a cyclic shape via an α-1,4-glucoside bond. Examples of the cyclodextrin include α-cyclodextrin (number of glucose units: 6), β-cyclodextrin (number of glucose units: 7), and γ-cyclodextrin (number of glucose units: 8), and α-cyclodextrin is preferable. As the cyclodextrin, one type may be used solely, and two or more types may be used in combination.

The linear molecule is preferably a linear molecule capable of piercing through the cyclic structure of the cyclodextrin so that the cyclic structure of the cyclodextrin is rotatable around the linear molecule. Examples of the linear molecule include polyalkylene, polyester, polyether, and polyacrylic acid. Among them, polyether is preferable, and polyethylene glycol is particularly preferable. Polyethylene glycol has less steric hindrance, and thus is easily included in the cyclic structure of the cyclodextrin in a manner of piercing through the cyclic structure of the cyclodextrin.

The weight average molecular weight of the linear molecule is preferably 5,000 or more, more preferably 6,000 or more, and is preferably 100,000 or less, more preferably 80,000 or less.

The linear molecule preferably has functional groups at both terminals thereof. When the linear molecule has the functional group, the linear molecule easily reacts with the blocking group. Examples of the functional group include a hydroxyl group, a carboxyl group, an amino group, and a thiol group.

The blocking group is not particularly limited, as long as it is located at both terminals of the linear molecule to prevent the disassociation of the cyclodextrin from the linear molecule. Examples of the method for preventing the disassociation include a method of using a bulky blocking group to physically prevent the disassociation, and a method of using an ionic blocking group to electrostatically prevent the disassociation. Examples of the bulky blocking group include a cyclodextrin and an adamantyl group. The number of the cyclodextrins penetrated by the linear molecule preferably ranges from 0.06 to 0.61, more preferably ranges from 0.11 to 0.48, and even more preferably ranges from 0.24 to 0.41, if the maximum number thereof is deemed as 1. This is because if the number is less than 0.06, the pulley effect may not be exerted, and if the number exceeds 0.61, the cyclodextrins are very densely located, so that the movability of the cyclodextrin may decrease.

As the polyrotaxane, a polyrotaxane having at least a part of hydroxyl groups of the cyclodextrin being modified with a caprolactone chain, is preferred. Modifying with the caprolactone alleviates the steric hindrance between the polyrotaxane and the polyisocyanate, thereby enhancing the reaction efficiency with the polyisocyanate.

As the above modification, for example, the hydroxyl groups of the cyclodextrin are treated with propylene oxide to hydroxylpropylate the cyclodextrin, and then ε-caprolactone is added to perform ring-opening polymerization. As a result of this modification, the caprolactone chain —(CO(CH₂)₅O)_(n)H (n is a natural number of 1 to 100) is linked to the exterior side of the cyclic structure of the cyclodextrin via —O—C₃H₆—O— group. “n” represents the degree of polymerization, and is preferably a natural number of 1 to 100, more preferably a natural number of 2 to 70, and even more preferably a natural number of 3 to 40. At another terminal of the caprolactone chain, a hydroxyl group is formed through the ring-opening polymerization.

The ratio of the hydroxyl groups modified with the caprolactone chain to all the hydroxyl groups (100 mole %) included in the cyclodextrin before the modification is preferably 2 mole % or more, more preferably 5 mole % or more, even more preferably 10 mole % or more. If the ratio of the hydroxyl groups modified with the caprolactone chain falls within the above range, the hydrophobicity of the polyrotaxane increases, and the reactivity with the polyisocyanate increases.

The hydroxyl value of the polyrotaxane is preferably 10 mg KOH/g or more, more preferably 15 mg KOH/g or more, even more preferably 20 mg KOH/g or more, and is preferably 400 mg KOH/g or less, more preferably 300 mg KOH/g or less, even more preferably 220 mg KOH/g or less, particularly preferably 180 mg KOH/g or less. If the hydroxyl value of the polyrotaxane falls within the above range, the reactivity with the polyisocyanate increases, and thus the durability of the paint film becomes more favorable. It is noted that the hydroxyl value can be measured according to JIS K 1557-1, for example, by an acetylation method.

The total molecular weight of the polyrotaxane is preferably 30,000 or more, more preferably 40,000 or more, and even more preferably 50,000 or more, and is preferably 3,000,000 or less, more preferably 2,500,000 or less, and even more preferably 2,000,000 or less, in a weight average molecular weight. If the weight average molecular weight is 30,000 or more, the paint film has sufficient strength, and if the weight average molecular weight is 3,000,000 or less, the paint film has sufficient flexibility and thus approach performance of the golf ball increases. It is noted that the weight average molecular weight of the polyrotaxane can be measured, for example, by gel permeation chromatography (GPC) using polystyrene as a standard substance, tetrahydrofuran as an eluant, and an organic solvent system GPC column (e.g., “Shodex (registered trademark) KF series” available from Showa Denko K.K.) as a column.

Specific examples of the polyrotaxane modified with the polycaprolactone include SeRM super polymer SH3400P, SH2400P, and SH1310P available from Advanced Softmaterials Inc.

The hydroxyl group modified vinyl chloride-vinyl acetate copolymer adjusts the adhesion of the paint film while maintaining the scuff resistance of the paint film. The hydroxyl group modified vinyl chloride-vinyl acetate copolymer is obtained, for example, by a method of copolymerizing vinyl chloride, vinyl acetate and a monomer having a hydroxyl group (e.g., polyvinyl alcohol, hydroxyalkyl acrylate), or by a method of partially or completely saponifying a vinyl chloride-vinyl acetate copolymer.

The hydroxyl value of the hydroxyl group modified vinyl chloride-vinyl acetate copolymer is preferably 20 mgKOH/g or more, more preferably 30 mgKOH/g or more, and even more preferably 40 mgKOH/g or more, and is preferably 200 mgKOH/g or less, more preferably 180 mgKOH/g or less, and even more preferably 160 mgKOH/g or less.

The amount of the vinyl chloride component in the hydroxyl group modified vinyl chloride-vinyl acetate copolymer is preferably 1 mass % or more, more preferably 20 mass % or more, even more preferably 50 mass % or more, and is preferably 99 mass % or less, more preferably 95 mass % or less. Specific examples of the hydroxyl group modified vinyl chloride-vinyl acetate copolymer include Solbin (registered trademark) A, Solbin AL, and Solbin TA3 available from Nissin Chemical Industry Co., Ltd.

The polyol component preferably contains the polyrotaxane. In this case, the amount of the polyrotaxane in the polyol component is preferably 30 mass % or more, more preferably 35 mass % or more, and even more preferably 40 mass % or more.

The polyol component preferably contains a polycaprolactone polyol. The mass ratio (polycaprolactone polyol/polyrotaxane) of the polycaprolactone polyol to the polyrotaxane is preferably 1/100 or more, more preferably 5/95 or more, and even more preferably 10/90 or more, and is preferably 90/10 or less, more preferably 85/15 or less, and even more preferably 80/20 or less.

The polyol component also preferably contains the hydroxyl group modified vinyl chloride-vinyl acetate copolymer. When the polyol component contains the hydroxyl group modified vinyl chloride-vinyl acetate copolymer, the amount of the hydroxyl group modified vinyl chloride-vinyl acetate copolymer in the polyol component is preferably 4 mass % or more, more preferably 8 mass % or more, and is preferably 50 mass % or less, more preferably 45 mass % or less.

The molar ratio (NCO group/OH group) of the isocyanate group (NCO group) included in (A) the polyisocyanate composition to the hydroxyl group (OH group) included in (B) the polyol composition is preferably 0.1 or more, more preferably 0.5 or more, and even more preferably 1.1 or more. If the molar ratio (NCO group/OH group) is 0.1 or more, the curing reaction sufficiently proceeds. In addition, if the molar ratio (NCO group/OH group) is excessively large, the amount of the isocyanate group is excessive, and the obtained paint film not only becomes hard and fragile but also has deteriorated poor appearance. Thus, the molar ratio (NCO group/OH group) is preferably 2.0 or less, more preferably 1.7 or less, and even more preferably 1.5 or less. It is noted that the reason why the appearance of the obtained paint film deteriorates if the amount of the isocyanate group in the paint becomes excessive is that an excessive amount of the isocyanate group may promote a reaction between the moisture in air and the isocyanate group, thereby generating a lot of carbon dioxide gas.

(A) The polyisocyanate composition and (B) the polyol composition may contain a solvent. The solvent may be water or an organic solvent, and is preferably the organic solvent. Examples of the organic solvent include toluene, isopropyl alcohol, xylene, methylethyl ketone, methylethylisobutyl ketone, ethylene glycol monomethyl ether, ethylbenzene, propylene glycol monomethyl ether, isobutyl alcohol, and ethyl acetate. It is noted that the solvent may be added in either of (A) the polyisocyanate composition and (B) the polyol composition, and in light of uniformly performing the curing reaction, the solvent is preferably added in the polyol composition and the polyisocyanate composition, respectively.

(A) The polyisocyanate composition and (B) the polyol composition may contain a catalyst for the curing reaction. Examples of the catalyst include a monoamine such as triethyl amine and N,N-dimethylcyclohexylamine; a polyamine such as N,N,N′,N′-tetramethylethylene diamine and N,N,N′,N″,N″-pentamethyldiethylene triamine; a cyclic diamine such as 1,8-diazabicyclo[5.4.0]-7-undecene (DBU) and triethylene diamine; and a tin catalyst such as dibutyl tin dilaurate and dibutyl tin diacetate. These catalysts may be used solely, or two or more of the catalysts may be used in combination. Among them, the tin catalyst such as dibutyl tin dilaurate and dibutyl tin diacetate is preferable, and dibutyl tin dilaurate is particularly preferable.

(A) The polyisocyanate composition and (B) the polyol composition may further contain additives generally contained in a paint for a golf ball, such as a filler, an ultraviolet absorber, an antioxidant, a light stabilizer, a fluorescent brightener, an anti-blocking agent, a leveling agent, a slip agent, and a viscosity modifier, where necessary.

The outermost paint film layer is preferably formed from a paint containing (A) the polyisocyanate composition and (B) the polyol composition. Examples of the paint include a so-called two-component curing type urethane paint containing the polyol as a base material and the polyisocyanate as a curing agent.

(Properties of the Outermost Paint Film Layer)

The dynamic friction coefficient of the outermost paint film layer is preferably 0.75 or more, more preferably 0.77 or more, even more preferably 0.80 or more, and most preferably 0.82 or more, and is preferably 1.5 or less, more preferably 1.4 or less, and even more preferably 1.3 or less. If the dynamic friction coefficient is less than 0.75, the spin rate on approach shots under a condition that there is grass between the golf ball and the club face decreases, and if the dynamic friction coefficient is more than 1.5, the paint film has strong adhesiveness and thus is easily stained.

The peak temperature of the loss tangent (tan δ) of the outermost paint film layer is preferably −40° C. or more, more preferably −30° C. or more, and even more preferably −20° C. or more, and is preferably 40° C. or less, more preferably 30° C. or less, and even more preferably 20° C. or less. If the peak temperature of the loss tangent (tan δ) is −40° C. or more, the paint film has low adhesiveness and thus is hard to be stained, and if the peak temperature of the loss tangent (tan δ) is 40° C. or less, the paint film has a high dynamic friction coefficient and thus the spin rate on approach shots under a condition that there is grass between the golf ball and the club face increases.

The storage modulus (E′) of the outermost paint film layer at the temperature of 24° C. is preferably 10 MPa or more, more preferably 15 MPa or more, and even more preferably 20 MPa or more, and is preferably 100 MPa or less, more preferably 80 MPa or less, and even more preferably 60 MPa or less. If the storage modulus (E′) is 10 MPa or more, the paint film has low adhesiveness and thus is hard to be stained, and if the storage modulus (E′) is 100 MPa or less, the paint film has a high dynamic friction coefficient and thus the spin rate on approach shots under a condition that there is grass between the golf ball and the club face increases.

The loss modulus (E″) of the outermost paint film layer at the temperature of 24° C. is preferably 0.1 MPa or more, more preferably 0.5 MPa or more, and even more preferably 1.0 MPa or more, and is preferably 80 MPa or less, more preferably 65 MPa or less, and even more preferably 50 MPa or less. If the loss modulus (E″) is 0.1 MPa or more, the paint film has low adhesiveness and thus is hard to be stained, and if the loss modulus (E″) is 80 MPa or less, the paint film has a high dynamic friction coefficient and thus the spin rate on approach shots under a condition that there is grass between the golf ball and the club face increases.

The 10% elastic modulus of the outermost paint film layer is preferably 1 kgf/cm² (0.10 MPa) or more, more preferably 3 kgf/cm² (0.29 MPa) or more, and even more preferably 5 kgf/cm² (0.49 MPa) or more, and is preferably 50 kgf/cm² (4.9 MPa) or less, more preferably 40 kgf/cm² (3.9 MPa) or less, and even more preferably 30 kgf/cm² (2.9 MPa) or less. If the 10% elastic modulus of the outermost paint film layer is 1 kgf/cm² or more, the paint film has low adhesiveness and thus is hard to be stained, and if the 10% elastic modulus of the outermost paint film layer is 50 kgf/cm² or less, the paint film has a high dynamic friction coefficient and thus the spin rate on approach shots under a condition that there is grass between the golf ball and the club face increases.

The thickness of the outermost paint film layer is preferably 5 μm or more, more preferably 7 μm or more, and is preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. If the thickness of the outermost paint film layer falls within the above range, the outermost paint film layer has better appearance, and the golf ball has better abrasion resistance and better approach performance.

The methods for measuring the dynamic friction coefficient, loss tangent, storage modulus, loss modulus and thickness of the outermost paint film layer will be described later.

When the paint film has a multiple layered structure, examples of the base resin constituting the paint film layer other than the outermost paint film layer include a urethane resin, an epoxy resin, an acrylic resin, a vinyl acetate resin, and a polyester resin, and among them, the urethane resin is preferable. The paint film layer other than the outermost paint film layer may be formed from a paint same as or different from the paint for forming the outermost paint film layer.

(Formation of Paint Film)

The paint film can be formed by applying a paint on the surface of the golf ball body. The method of applying the paint is not particularly limited, a conventional method can be adopted, and examples thereof include a spray coating and electrostatic coating.

In the case of performing the spray coating with an air gun, (A) the polyisocyanate composition and (B) the polyol composition are fed with respective pumps and continuously mixed with a line mixer located in the stream line just before the air gun, and the obtained mixture is air-sprayed. Alternatively, the polyol and the polyisocyanate are air-sprayed respectively with an air spray system provided with a device for controlling the mixing ratio thereof. The paint application may be conducted by spraying the paint one time or overspraying the paint multiple times.

The paint applied on the golf ball body can be dried, for example, at a temperature in a range of from 30° C. to 70° C. for 1 hour to 24 hours, to form the paint film.

(Golf Ball Body)

The golf ball according to the present invention is not particularly limited, as long as it is a golf ball comprising a golf ball body and a paint film formed on a surface of the golf ball body. The construction of the golf ball body is not particularly limited, and the golf ball body may be a one-piece golf ball, a two-piece golf ball, a multi-piece golf ball such as a three-piece golf ball, a four-piece golf ball, a five-piece golf ball and a golf ball comprising more than five pieces, or a wound golf ball. The present invention can be applied appropriately to any one of the above golf balls.

(Core)

The one-piece golf ball body and the core used for a wound golf ball, two-piece golf ball and multi-piece golf ball will be explained.

The one-piece golf ball body or the core may be formed from a conventionally known rubber composition (hereinafter sometimes simply referred to as “core rubber composition”). For example, the one-piece golf ball body or the core may be formed by heat pressing a rubber composition containing a base rubber, a co-crosslinking agent and a crosslinking initiator.

As the base rubber, particularly preferable is a high cis-polybutadiene having a cis-bond in a proportion of 40 mass % or more, more preferably 70 mass % or more, and even more preferably 90 mass % or more in view of its advantageous resilience. As the co-crosslinking agent, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms or a metal salt thereof is preferable, and a metal salt of acrylic acid or a metal salt of methacrylic acid is more preferable. As the metal constituting the metal salt, zinc, magnesium, calcium, aluminum or sodium is preferable, and zinc is more preferable. The amount of the co-crosslinking agent is preferably 20 parts by mass or more and 50 parts by mass or less with respect to 100 parts by mass of the base rubber. In the case that the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is used as the co-crosslinking agent, a metal compound (e.g. magnesium oxide) is preferably used in combination. As the crosslinking initiator, an organic peroxide is preferably used. Specific examples of the organic peroxide include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Among them, dicumyl peroxide is preferably used. The amount of the crosslinking initiator is preferably 0.2 part by mass or more, more preferably 0.3 part by mass or more, and is preferably 3 parts by mass or less, more preferably 2 parts by mass or less, with respect to 100 parts by mass of the base rubber.

In addition, the core rubber composition may further contain an organic sulfur compound. As the organic sulfur compound, diphenyl disulfides (e.g. diphenyl disulfide, bis(pentabromophenyl)disulfide), thiophenols or thionaphthols (e.g. 2-thionaphthol) are preferably used. The amount of the organic sulfur compound is preferably 0.1 part by mass or more, more preferably 0.3 part by mass or more, and is preferably 5.0 parts by mass or less, more preferably 3.0 parts by mass or less, with respect to 100 parts by mass of the base rubber. The core rubber composition may further contain a carboxylic acid and/or a salt thereof. As the carboxylic acid and/or the salt thereof, a carboxylic acid having 1 to 30 carbon atoms and/or a salt thereof is preferable. As the carboxylic acid, an aliphatic carboxylic acid or an aromatic carboxylic acid (e.g. benzoic acid) may be used. The amount of the carboxylic acid and/or the salt thereof is preferably 1 part by mass or more and 40 parts by mass or less with respect to 100 parts by mass of the base rubber.

The core rubber composition may further contain a weight adjusting agent such as zinc oxide and barium sulfate, an antioxidant, or a colored powder, in addition to the base rubber, the co-crosslinking agent, the crosslinking initiator, and the organic sulfur compound. The molding conditions for heat pressing the core rubber composition may be appropriately set depending on the rubber formulation. Generally, the heat pressing is preferably carried out at 130° C. to 200° C. for 10 to 60 minutes, or carried out in a two-step heating of heating at 130° C. to 150° C. for 20 to 40 minutes followed by heating at 160° C. to 180° C. for 5 to 15 minutes.

(Cover)

The golf ball body preferably comprises a core and a cover covering the core. In this case, the hardness of the cover is preferably 60 or less, more preferably 55 or less, even more preferably 50 or less, and most preferably 45 or less in Shore D hardness. If the hardness of the cover is 60 or less in Shore D hardness, the spin rate is further increased. The lower limit of the hardness of the cover is not particularly limited, and is preferably 10, more preferably 15, and even more preferably 20 in Shore D hardness. The hardness of the cover is a slab hardness of the cover composition molded into a sheet shape.

The thickness of the cover is preferably 0.1 mm or more, more preferably 0.2 mm or more, and even more preferably 0.3 mm or more, and is preferably 1.0 mm or less, more preferably 0.9 mm or less, and even more preferably 0.8 mm or less. If the thickness of the cover is 0.1 mm or more, the shot feeling of the golf ball is better, and if the thickness of the cover is 1.0 mm or less, the resilience of the golf ball is maintained.

The resin component constituting the cover is not particularly limited, and examples thereof include various resins such as an ionomer resin, a polyester resin, a urethane resin and a polyamide resin; a thermoplastic polyamide elastomer having a trade name of “Pebax (registered trademark) (e.g. “Pebax 2533”)” available from Arkema Inc.; a thermoplastic polyester elastomer having a trade name of “Hytrel (registered trademark) (e.g. “Hytrel 3548” and “Hytrel 4047”)” available from Du Pont-Toray Co., Ltd.; a thermoplastic polyurethane elastomer having a trade name of “Elastollan (registered trademark) (e.g. “Elastollan XNY97A”)” available from BASF Japan Ltd.; and a thermoplastic styrene elastomer having a trade name of “Rabalon (registered trademark)” and a thermoplastic polyester elastomer having a trade name of “Primalloy” both available from Mitsubishi Chemical Corporation. These cover materials may be used solely, or two or more of these cover materials may be used in combination.

Among them, the resin component constituting the cover is preferably the polyurethane resin or the ionomer resin, particularly preferably the polyurethane resin. When the resin component constituting the cover includes the polyurethane resin, the amount of the polyurethane resin in the resin component is preferably 50 mass % or more, more preferably 70 mass % or more, and even more preferably 90 mass % or more. When the resin component constituting the cover includes the ionomer resin, the amount of the ionomer resin in the resin component is preferably 50 mass % or more, more preferably 70 mass % or more, and even more preferably 90 mass % or more.

The polyurethane may be either a thermoplastic polyurethane or a thermosetting polyurethane. The thermoplastic polyurethane is a polyurethane exhibiting plasticity by heating and generally means a polyurethane having a linear chain structure of a high-molecular weight to a certain extent. On the other hand, the thermosetting polyurethane (two-component curing type polyurethane) is a polyurethane obtained by polymerization through a reaction between a low-molecular weight urethane prepolymer and a curing agent (chain extender) when molding the cover. The thermosetting polyurethane includes a polyurethane having a linear chain structure or a polyurethane having a three-dimensional crosslinked structure depending on the number of the functional group of the prepolymer or the curing agent (chain extender) to be used. As the polyurethane, the thermoplastic elastomer is preferable.

The cover may further contain a pigment component such as a white pigment (e.g. titanium oxide), a blue pigment and a red pigment, a weight adjusting agent such as calcium carbonate and barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material or a fluorescent brightener, or the like, in addition to the above resin component, as long as they do not impair the performance of the cover.

The method for molding the cover from the cover composition is not particularly limited, and examples thereof include a method of injection molding the cover composition directly onto the core; and a method of molding the cover composition into hollow shells, covering the core with a plurality of the hollow shells and compression molding the core with a plurality of the hollow shells (preferably a method of molding the cover composition into half hollow-shells, covering the core with two of the half hollow-shells and compression molding the core with two of the half hollow-shells). The golf ball body having the cover formed thereon is ejected from the mold, and as necessary, the golf ball body is preferably subjected to surface treatments such as deburring, cleaning, and sandblast. In addition, if desired, a mark may also be formed thereon.

The total number of the dimples formed on the cover is preferably 200 or more and 500 or less. If the total number of the dimples is less than 200, the dimple effect is hardly obtained, and if the total number of the dimples exceeds 500, the dimple effect is hardly obtained because the size of the respective dimples is small. The shape (shape in a plan view) of the dimples includes, for example, but is not limited to, a circle; a polygonal shape such as a roughly triangular shape, a roughly quadrangular shape, a roughly pentagonal shape, and a roughly hexagonal shape; and other irregular shape. The shape of the dimples may be employed solely, or two or more of the shapes may be employed in combination.

When the golf ball is a multi-piece golf ball such as a three-piece golf ball, a four-piece golf ball, a five-piece golf ball and a golf ball composed of more than five pieces, examples of the material for forming the intermediate layer disposed between the core and the outermost cover include a thermoplastic resin such as a polyurethane resin, an ionomer resin, a polyamide resin, and a polyethylene; a thermoplastic elastomer such as a styrene elastomer, a polyolefin elastomer, a polyurethane elastomer, and a polyester elastomer; and a cured product of a rubber composition. Herein, examples of the ionomer resin include a product obtained by neutralizing at least a part of carboxyl groups of a copolymer composed of ethylene and an α,β-unsaturated carboxylic acid with a metal ion; and a product obtained by neutralizing at least a part of carboxyl groups of a ternary copolymer composed of ethylene, an α,β-unsaturated carboxylic acid and an α,β-unsaturated carboxylic acid ester with a metal ion. The intermediate layer may further contain a weight adjusting agent such as barium sulfate and tungsten, an antioxidant, a pigment, and the like. It is noted that the intermediate layer is sometimes referred to as an inner cover layer or an outer core layer, depending on the construction of the golf ball.

The golf ball preferably has a diameter in a range from 40 mm to 45 mm. In light of satisfying the regulation of US Golf Association (USGA), the diameter is particularly preferably 42.67 mm or more. In light of prevention of air resistance, the diameter is more preferably 44 mm or less, even more preferably 42.80 mm or less. In addition, the golf ball according to the present invention preferably has a mass of 40 g or more and 50 g or less. In light of obtaining greater inertia, the mass is more preferably 44 g or more, even more preferably 45.00 g or more. In light of satisfying the regulation of USGA, the mass is particularly preferably 45.93 g or less.

When the golf ball has a diameter in a range from 40 mm to 45 mm, the compression deformation amount (shrinking amount along the compression direction) of the golf ball when applying a load from 98 N as an initial load to 1275 N as a final load to the golf ball is preferably 2.0 mm or more, more preferably 2.2 mm or more, and is preferably 4.0 mm or less, more preferably 3.5 mm or less. If the compression deformation amount is 2.0 mm or more, the golf ball is not excessively hard and thus the shot feeling thereof is better. On the other hand, if the compression deformation amount is 4.0 mm or less, the resilience is greater.

The FIGURE is a partially cutaway cross-sectional view showing a golf ball 1 according to one embodiment of the present invention. The golf ball 1 comprises a spherical core 2, a cover 3 covering the spherical core 2, and a paint film 4 formed on a surface of the cover 3. On the surface of the cover 3, a plurality of dimples 31 are formed. On the surface of the cover 3, a part other than the dimples 31 is a land 32.

EXAMPLES

Next, the present invention will be described in detail by way of examples. However, the present invention is not limited to the examples described below. Various changes and modifications without departing from the spirit of the present invention are included in the scope of the present invention.

[Evaluation Method] (1) Compression Deformation Amount (mm)

The deformation amount of the core along the compression direction (shrinking amount of the core along the compression direction), when applying a load from 98 N as an initial load to 1275 N as a final load to the core, was measured.

(2) Core Hardness (Shore C Hardness)

The hardness measured at the surface of the core was adopted as the surface hardness of the core. In addition, the core was cut into two hemispheres and the hardness measured at the central point of the obtained cut plane was adopted as the center hardness of the core. The hardness was measured with an automatic hardness tester (Digitest II available from Bareiss company) using a detector of “Shore C”.

(3) Slab Hardness (Shore D Hardness)

Sheets with a thickness of about 2 mm were produced by injection molding the resin composition. The sheets were stored at 23° C. for two weeks. At least three of these sheets were stacked on one another so as not to be affected by the measuring substrate on which the sheets were placed, and the hardness of the stack was measured with an automatic hardness tester (Digitest II available from Bareiss company) using a detector of “Shore D”.

(4) Thickness of Paint Film (μm)

The golf ball was cut into two hemispheres, and the cross section of the paint film on the hemisphere was observed with a microscope (VHX-1000 available from Keyence Corporation) to obtain the thickness of the paint film.

(5) Loss Tangent (Tan δ)

The storage modulus E′ (Pa), loss modulus E″ (Pa) and loss tangent (tan δ) of the paint film were measured under the following conditions.

Apparatus: Dynamic viscoelasticity measuring apparatus (Rheogel-E4000 available from UBM CO., Ltd.)

Test sample: A paint obtained by blending the polyisocyanate composition and the polyol composition was dried and cured at the temperature of 40° C. for 4 hours to prepare a paint film having a thickness ranging from 0.11 mm to 0.14 mm. The paint film was cut to prepare a test piece having a width of 4 mm and a distance between the clamps of 20 mm.

Measuring mode: tensile mode

Measuring temperature: −100° C. to 150° C.

Temperature increase rate: 4° C./min

Measuring date capturing interval: 4° C.

Oscillation frequency: 10 Hz

Measuring strain: 0.1%

(6) 10% Elastic Modulus of Paint Film

The tensile properties of the paint film were measured according to JIS K7161 (2014). Specifically, the polyisocyanate composition and the polyol composition were blended to prepare a paint, and the obtained paint was dried and cured at the temperature of 40° C. for 4 hours to prepare a paint film (thickness: 0.05 mm). The paint film was punched into the test piece type II (width of parallel part: 10 mm, gauge length: 50 mm) prescribed in JIS K7127 (1999), to prepare a test piece. The tensile test of the test piece was conducted with a precision universal tester (Autograph (registered trademark) available from Shimadzu Corporation) under testing conditions of a length between grips: 100 mm, a tensile speed: 50 mm/min and a testing temperature: 23° C., and the tensile stress at 10% strain (10% elastic modulus) was recorded.

(7) Spin Rate Under Dry Condition

A sand wedge (trade name: “CG 15 forged wedge”, loft angel: 52° available from Cleveland Golf Inc.) was installed on a swing machine available from Golf Laboratories, Inc. The golf ball was hit at a head speed of 16 m/s, and the spin rate (rpm) thereof was measured by continuously taking a sequence of photographs of the hit golf ball. The measurement was conducted ten times for each golf ball, and the average value thereof was adopted as the spin rate.

(8) Spin Rate Under a Condition that there is Grass Between the Golf Ball and the Club Face

A sand wedge (trade name: “CG 15 forged wedge”, loft angel: 52° available from Cleveland Golf Inc.) was installed on a swing machine available from Golf Laboratories, Inc. The golf ball was hit at a head speed of 16 m/s, and the spin rate (rpm) thereof was measured by continuously taking a sequence of photographs of the hit golf ball. It is noted that two leaves (length: about 3 cm) of wild grass were attached to the golf ball that was the testing object, and the golf ball was hit such that there was the wild grass between the club face and the golf ball. The measurement was conducted ten times for each golf ball, and the average value thereof was adopted as the spin rate.

(9) Dynamic Friction Coefficient

The dynamic friction coefficient of the golf ball was measured under the following conditions. It is noted that in the following conditions, the load is small and the moving speed is low, thus the dynamic friction coefficient of the outermost paint film layer can be measured.

Tester: Tribo Master TL201TS (available from Trinity-Lab Inc.)

Test sample: golf ball

Face plate: A plate made of stainless steel (HT1770 (size: 50 mm×150 mm× thickness 2 mm) available from Nippon Steel Nisshin Co., Ltd.) and having a face surface with an arithmetic mean roughness Ra of 2.9 μm and a maximum height mean value Ry of 21.7 μm was used.

Preparation method of face plate: The face plate was obtained by performing an air-blast treatment to the stainless steel. As the grinding material, a mixture of alumina powder (#60) and steel ball (ES300 available from Ervin industries Co., Ltd.) in a mixing ratio of 1:1 was used. The blasting conditions were: a distance of 10 cm between the face plate and the nozzle, and a pressure of 4 to 6 kg/cm² just before the nozzle.

Measuring method of Ra and Ry: Ra and Ry were measured with a surface roughness tester (SJ-301 available from Mitutoyo Corporation) by a method based on JIS B 0601-1994. Ra, Ry were each an average value of the values obtained at six measuring points. In addition, the cutoff value λc was 2.5 mm.

Temperature: 23° C.

Ball moving speed: 2 mm/s

Load: 1.96 N (200 gf)

Measurement method: The ball was fixed with a chuck, and allowed to move at a predetermined speed on a flat plate in a state that 200 gf of a load was applied on the ball, to measure the dynamic friction force.

Measuring item: dynamic friction (average value in the section of from 2-10 mm)

[Production of Golf Ball] 1. Production of Spherical Core

The rubber composition having the formulation shown in Table 1 was kneaded, and heat-pressed at the temperature of 150° C. for 19 min in upper and lower molds, each having a hemispherical cavity, to obtain a spherical core having a diameter of 39.7 mm. It is noted that the amount of barium sulfate was adjusted such that the ball had a mass of 45.6 g.

TABLE 1 Spherical core Rubber Polybutadiene rubber 100 composition Zinc acrylate 30.5 formulation Zinc oxide 10 (parts by Barium sulfate *1) mass) Bis(pentabromophenyl)disulfide 0.3 Dicumyl peroxide 0.7 Benzoic acid 2 Molding Temperature (° C.) 150 condition Time (min) 19 Core Compression deformation amount (mm) 3.3 properties Center hardness (Shore C) 53 Surface hardness (Shore C) 80 Hardness difference (Surface-center) 27 (Shore C) *1) Appropriate amount Polybutadiene rubber: “BR730 (high-cis polybutadiene)” available from JSR Corporation Zinc acrylate: “ZN-DA90S” available from Nihon Jyoryu Kogyo Co., Ltd. Zinc oxide: “Ginrei R” available from Toho Zinc Co., Ltd. Barium sulfate: “Barium Sulfate BD” available from Sakai Chemical Industry Co., Ltd. Bis(pentabromophenyl)disulfide: available from Kawaguchi Chemical Industry Co., Ltd. Dicumyl peroxide: “Percumyl (register trademark) D” available from NOF Corporation Benzoic acid: available from Emerald Kalama Chemical Co., Ltd.

2. Preparation of Intermediate Layer Composition and Cover Composition

According to the formulations shown in Tables 2 and 3, the materials were mixed with a twin-screw kneading extruder to prepare the intermediate layer composition and the cover composition in a pellet form. The extruding conditions were a screw diameter of 45 mm, a screw rotational speed of 200 rpm, and a screw L/D=35, and the mixture was heated to 160° C. to 230° C. at the die position of the extruder.

TABLE 2 Intermediate layer composition Himilan AM7329 55 Himilan 1555 45 Barium sulfate Appropriate amount Titanium dioxide 3 Slab hardness (Shore D) 62

Himilan (registered trademark) AM7329: zinc ion-neutralized ethylene-methacrylic acid copolymer ionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd.

Himilan 1555: sodium ion-neutralized ethylene-methacrylic acid copolymer ionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd.

TABLE 3 Cover composition Elastollan NY80A 100 Titanium dioxide 4 Ultramarine blue 0.04 Slab hardness (Shore D) 27

Elastollan (registered trademark) NY80A: thermoplastic polyurethane elastomer available from BASF Japan Ltd.

3. Molding of Intermediate Layer

The intermediate layer composition obtained above was directly injection molded onto the spherical core obtained above to form the intermediate layer (thickness: 1.0 mm) covering the spherical core.

4. Production of Reinforcing Layer

A reinforcing layer composition (trade name “Polin (registered trademark) 750LE” available from Shinto Paint Co., Ltd.) having a two-component curing type epoxy resin as the base resin was prepared. The base agent contains a bisphenol A type solid epoxy resin in an amount of 30 parts by mass, and a solvent in an amount of 70 parts by mass. The curing agent contains a modified polyamide amine in an amount of 40 parts by mass, titanium dioxide in an amount of 5 parts by mass, and a solvent in an amount of 55 parts by mass. The mass ratio of the base agent to the curing agent was 1/1. The reinforcing layer composition was applied to the surface of the intermediate layer with an air gun, and kept for 12 hours in an atmosphere of 23° C., to form the reinforcing layer. The thickness of the reinforcing layer was 7 μm.

5. Molding of Cover

The cover composition in the pellet form was charged into each of the depressed part of the lower mold for molding half shells, and a pressure was applied to mold half shells. The spherical body having the reinforcing layer formed thereon was concentrically covered with two of the half shells. The spherical body and the half shells were charged into a final mold provided with a plurality of pimples on the cavity surface. The cover (thickness: 0.5 mm) was formed by compression molding to obtain golf ball bodies. A plurality of dimples having an inverted shape of the pimples were formed on the cover.

6. Formation of Paint Film

Materials shown in Tables 4 and 5 were blended to prepare paint compositions No. 1 to No. 14. The surface of the golf ball bodies obtained above was treated with sandblast and marked. The paint was applied with a spray gun, and dried for 24 hours in an oven at 40° C. to obtain golf balls having a diameter of 42.7 mm and a mass of 45.6 g.

The application of the paint was conducted as follows. The golf ball body was placed in a rotating member provided with a prong, and the rotating member was allowed to rotate at 300 rpm. The application of the paint was conducted by spacing a spray distance (7 cm) between the air gun and the golf ball body, and moving the air gun in an up and down direction. The painting interval in the overpainting operation was set to 1.0 second. The application of the paint was conducted under the spraying conditions of overpainting operation: twice, spraying air pressure: 0.15 MPa, compressed air tank pressure: 0.10 MPa, painting time per one application: one second, atmosphere temperature: 20° C. to 27° C., and atmosphere humidity: 65% or less. Evaluation results regarding the spin performance of the obtained golf balls are shown in Tables 4 and 5.

TABLE 4 Golf ball No. 1 2 3 4 5 6 7 Paint Polyol composition Polyrotaxane 36 36 30 50 70 70 100 (parts by mass) Polycaprolactone polyol 35 35 70 50 17 17 — Vinyl chloride-vinyl acetate-vinyl 29 29 — — 13 13 — alcohol copolymer Polyisocyanate Adduct-modified product of HDI 50 100 100 100 50 100 100 composition Biuret-modified product of HDI — — — — — — — (parts by mass) Isocyanurate-modified product of HDI 50 — — — 50 — — Isocyanurate-modified product of IPDI — — — — — — — Polyisocyanate composition/polyol composition blending 1.2/1.0 1.2/1.0 1.2/1.0 1.2/1.0 1.2/1.0 1.2/1.0 1.2/1.0 ratio (NCO/OH molar ratio) Paint film Thickness (μm) 10 10 10 10 10 10 10 Peak temperature of loss tangent (tanδ) (° C.) 15 −4 −6.5 −8 2 −15 −16 Peak height of loss tangent (tanδ) 0.77 0.78 0.71 0.71 0.74 0.75 0.65 Storage modulus E′ at 24° C. (MPa) 35 26 12 9 16 13 11 Loss modulus E″ at 24° C. (MPa) 35 13 1 0.8 4 3 0.5 10% Elastic modulus (kgf/cm²) 8.8 4.5 6.3 6 6 4.5 4 Dynamic friction coefficient of outermost paint film layer 0.82 0.92 0.87 0.89 0.87 0.91 1.1 Evaluation Spin rate under a dry condition (rpm) 4650 4700 4700 4680 4600 4600 4600 Spin rate under a condition that there is grass between the 3600 3700 3700 3700 3650 3800 3900 golf ball and the club face (rpm) Spin rate retention ratio under a condition that there is grass 77.4 78.7 78.7 79.1 79.3 82.6 84.8 between the golf ball and the club face (%)

TABLE 5 Golf ball No. 8 9 10 11 12 13 14 Paint Polyol composition Polyrotaxane 50 50 50 50 0 50 50 (parts by mass) Polycaprolactone polyol 28 28 28 28 55 28 28 Vinyl chloride-vinyl acetate-vinyl 22 22 22 22 45 22 22 alcohol copolymer Polyisocyanate Adduct-modified product of HDI 100 50 100 100 — — — composition Biuret-modified product of HDI — — — — — — — (parts by mass) Isocyanurate-modified product of HDI — 50 — — 100 100 70 Isocyanurate-modified product of IPDI — — — — — — 30 Polyisocyanate composition/polyol composition blending 1.2/1.0 1.2/1.0 1.4/1.0 1.6/1.0 1.2/1.0 1.2/1.0 1.2/1.0 ratio (NCO/OH molar ratio) Paint film Thickness (μm) 10 10 10 10 10 10 10 Peak temperature of loss tangent (tanδ) (° C.) −4 0 −5 −6 52 16 48 Peak height of loss tangent (tanδ) 0.77 0.76 0.75 0.73 0.79 0.75 0.56 Storage modulus E′ at 24° C. (MPa) 20 24 21 20 978 39 523 Loss modulus E″ at 24° C. (MPa) 2 7 3 2 126 27 168 10% Elastic modulus (kgf/cm²) 4.5 7 3.5 2.5 34.6 8.2 20 Dynamic friction coefficient of outermost paint film layer 0.87 0.83 0.89 0.90 0.61 0.75 0.72 Evaluation Spin rate under a dry condition (rpm) 4650 4650 4650 4650 4720 4650 4700 Spin rate under a condition that there is grass between the 3700 3650 3750 3780 3100 3400 3200 golf ball and the club face (rpm) Spin rate retention ratio under a condition that there is grass 79.6 78.5 80.6 81.3 65.7 73.1 68.1 between the golf ball and the club face (%)

The materials used in Tables 4 and 5 are shown as below.

Polyol Composition

-   -   Polyrotaxane: “SeRM (registered trademark) super polymer SH3400P         (a polyrotaxane having at least a part of hydroxyl groups of the         cyclodextrin being modified with a caprolactone chain via         —O—C₃H₆—O— group, linear molecule of the polyrotaxane:         polyethylene glycol, blocking group of the polyrotaxane:         adamantyl group; molecular weight of the linear molecule:         35,000, hydroxyl value of the polyrotaxane: 72 mg KOH/g, total         molecular weight of the polyrotaxane: 700,000 in weight average         molecular weight” available from Advanced Softmaterials Inc.     -   Polycaprolactone polyol: Placcel 308 available from Daicel         Chemical Industries, Ltd.     -   Modified product of Vinyl chloride-vinyl acetate copolymer:         Solbin AL (hydroxyl group modified vinyl chloride-vinyl acetate         copolymer) available from Nissin Chemical Industry Co., Ltd.     -   Modified silicone: DBL-C31 available from Gelest, Inc.

Polyisocyanate composition

-   -   Adduct-modified product of HDI: adduct-modified product of         hexamethylene diisocyanate (Duranate (registered trademark)         E402-80B (NCO amount: 7.3%) available from Asahi Kasei Chemicals         Corporation)     -   Biuret-modified product of HDI: biuret-modified product of         hexamethylene diisocyanate (Duranate (registered trademark)         21S-75E (NCO amount: 15.5%) available from Asahi Kasei Chemicals         Corporation)     -   Isocyanurate-modified product of HDI: isocyanurate-modified         product of hexamethylene diisocyanate (Duranate TKA-100 (NCO         amount: 21.7%) available from Asahi Kasei Chemicals Corporation)     -   Isocyanurate-modified product of IPDI: isocyanurate-modified         product of isophorone diisocyanate (VESTANAT (registered         trademark) T1890 (NCO amount: 12.0%) available from Degussa Co.,         Ltd.)

The golf balls No. 1 to 11 are the cases that the base resin of the outermost paint film layer contains a polyurethane, and the dynamic friction coefficient of the outermost paint film layer is controlled in a range of from 0.75 to 1.5. These golf balls No. 1 to 11 are excellent in both the approach spin performance under a dry condition and the approach spin performance under a condition that there is grass between the golf ball and the club face.

The golf balls No. 12 to 14 are the cases that the dynamic friction coefficient of the outermost paint film layer is less than 0.82. These golf balls No. 12 to 14 are excellent in the approach spin performance under a dry condition, but is poor in the approach spin performance under a condition that there is grass between the golf ball and the club face.

This application is based on Japanese patent application No. 2018-207335 filed on Nov. 2, 2018, the contents of which are hereby incorporated by reference. 

1. A golf ball comprising a golf ball body and a paint film formed on a surface of the golf ball body, wherein the paint film is composed of at least one layer, and an outermost paint film layer located at the outermost layer of the golf ball contains, as a base resin, a polyurethane obtained by a reaction between (A) a polyisocyanate composition and (B) a polyol composition, and the outermost paint film layer has a dynamic friction coefficient of from 0.75 to 1.5 measured with a dynamic friction tester under following conditions: <measurement conditions> moving speed: 2 mm/s load: 1.96 N measuring item: dynamic friction average value in a moving distance of from 2 mm to 10 mm.
 2. The golf ball according to claim 1, wherein (B) the polyol composition contains, as a polyol component, a polyrotaxane having a cyclodextrin, a linear molecule piercing through the cyclic structure of the cyclodextrin, and blocking groups located at both terminals of the linear molecule to prevent disassociation of the cyclodextrin wherein at least a part of hydroxyl groups of the cyclodextrin is modified with a caprolactone chain via a —O—C₃H₆—O— group, and an amount of the polyrotaxane in the polyol component is 30 mass % or more.
 3. The golf ball according to claim 1, wherein (A) the polyisocyanate composition contains an adduct-modified product of hexamethylene diisocyanate as a polyisocyanate component, and an amount of the adduct-modified product of hexamethylene diisocyanate in the polyisocyanate component is 30 mass % or more.
 4. The golf ball according to claim 1, wherein (A) the polyisocyanate composition contains an adduct-modified product of hexamethylene diisocyanate and an isocyanurate-modified product of hexamethylene diisocyanate as a polyisocyanate component.
 5. The golf ball according to claim 1, wherein the outermost paint film layer has the dynamic friction coefficient of from 0.75 to 1.3.
 6. The golf ball according to claim 2, wherein (B) the polyol composition further contains a polycaprolactone polyol as the polyol component.
 7. The golf ball according to claim 6, wherein a mass ratio (polycaprolactone polyol/polyrotaxane) of the polycaprolactone polyol to the polyrotaxane ranges from 1/100 to 90/10.
 8. The golf ball according to claim 2, wherein (B) the polyol composition further contains a hydroxyl group modified vinyl chloride-vinyl acetate copolymer as the polyol component.
 9. The golf ball according to claim 8, wherein an amount of the hydroxyl group modified vinyl chloride-vinyl acetate copolymer in the polyol component ranges from 4 mass % to 50 mass %.
 10. The golf ball according to claim 4, wherein a mass ratio (adduct-modified product/isocyanurate-modified product) of the adduct-modified product of hexamethylene diisocyanate to the isocyanurate-modified product of hexamethylene diisocyanate is 0.43 or more.
 11. The golf ball according to claim 4, wherein a total amount of the adduct-modified product of hexamethylene diisocyanate and the isocyanurate-modified product of hexamethylene diisocyanate in the polyisocyanate component ranges from 70 mass % to 100 mass %.
 12. The golf ball according to claim 1, wherein a loss tangent (tan δ) of the outermost paint film layer has a peak temperature in a range of from −40° C. to 40° C.
 13. The golf ball according to claim 1, wherein a storage modulus (E′) of the outermost paint film layer at the temperature of 24° C. ranges from 10 MPa to 100 MPa.
 14. The golf ball according to claim 1, wherein a loss modulus (E″) of the outermost paint film layer at the temperature of 24° C. ranges from 0.1 MPa to 80 MPa.
 15. The golf ball according to claim 1, wherein a 10% elastic modulus of the outermost paint film layer ranges from 1 kgf/cm² to 50 kgf/cm². 